This invention relates to a process for producing sulfurized nickel catalysts.
Nickel catalysts, the activity of which is modified by the presence of sulfur, have many uses, particularly for catalyzing selective hydrogenation and isomerization reactions. In particular, such sulfurized nickel catalysts are used for partial and selective hydrogenation of unsaturated fatty acids and their derivatives, e.g., esters thereof, such as triglycerides or fats. Some vegetable oils, such as soybean oil, contain compounds having several double bonds (polyenic compounds), for instance, three double bonds (trienic compounds) and two double bonds (dienic compounds), in admixture with compounds having only one double bond (monoenic compounds) and saturated compounds. For some uses, for example, for the manufacture of confectionaries, icing fats and the like, these oils must be hardened. However, the hardened product must fulfill a number of conditions; it must remain relatively hard up to a temperature in the range of from about 20.degree. to about 40.degree. C., but it must melt completely in the mouth. For these reasons, the hydrogenation must be highly selective and isomerization must also take place during this hydrogenation. It is a prime importance:
(1) TO REDUCE THE CONTENT OF UNSTABLE POLYENIC COMPOUNDS OF THE OIL;
(2) TO LIMIT THE FORMATION OF SATURATED COMPOUNDS DURING THE HYDROGENATION, SINCE THESE SATURATED COMPOUNDS EXHIBIT TOO HIGH OF A MELTING POINT;
(3) TO LIMIT THE FORMATION OF UNSTABLE CONJUGATED DIENIC COMPOUNDS; AND,
(4) TO FAVOR THE ISOMERIZATION TO TRANS-ISOMERS, WHICH HAVE A HIGHER MELTING POINT THAN THE CIS-ISOMERS. For instance, elaidic acid (the trans-isomer) has a melting point of 46.5.degree. C., while oleic acid (the cis-isomer) has a melting point of 13.4.degree. C. (.alpha. form) or 16.3.degree. C. (.beta. form).
This isomerization, which is also called elaidization, does not take place in the presence of nickel catalysts free of sulfur (U.S. Pat. No. 2,123,342).
Heretofore, numerous sulfurized nickel catalysts and processes for manufacturing these catalysts have been proposed for carrying out these selective hydrogenation and isomerization reactions.
However, these catalysts have certain drawbacks. In fact, a sulfurized nickel catalyst is a nickel catalyst which is poisoned by the presence of sulfur. It results that a sulfurized catalyst is generally less active; hard fats can be produced from oils having a high iodine value (that is, a high degree of unsaturation), but the reaction rate is very low. Many attempts have been made to increase the reaction rate, but they lead to a poor selectivity of the reaction; the content of isomerized products is too low and the resulting hardened fat does not fulfill the requirements for the intended use in foods. A valuable catalyst for hardening oils must filfill a number of conditions, (activity, selectivity and isomerization power) which are somewhat conflicting.
Moreover, many methods for producing the sulfurized nickel catalysts give erratic results: the metallic catalysts are irregularly sulfurized and their activity varies widely. Furthermore, some previously proposed methods require a close control of working conditions. For instance, a sulfurized nickel catalyst can be prepared by reaction between a nickel catalyst and H.sub.2 S. However, the reaction is so exothermic that provision must be made for removing the reaction heat (Kirkpatrick "Nickel Sulfide Catalysts," Advances in Catalysis, Vol. III, p. 329, 1951).